Is your basement or garage exceptionally chilly' An uninsulated concrete floor may be a large part of the problem. Insulating a concrete floor can save you money on your heating bill, making your garage or basement warmer and, by extension, the rest of your house.
In addition to offering insulation and energy savings, the plywood subfloor described here can provide the perfect foundation for finish flooring such as carpet, adding warmth in look as well as in feel.
To prepare, first evaluate the condition of your concrete floor. If you know you have issues with moisture, tend to those problems before installing your insulation. Also, check your floor for smoothness and level; you may need to grind down significantly high spots to ensure an even surface.
Measure the distance between the slab and ceiling; you will want to leave approximately 7 feet, 6 inches of clearance between the ceiling and the surface of the finished floor to comply with building codes. The subfloor will take up about 2 inches, and any additional floor covering will also reduce your clearance, so plan accordingly. Finally, clean the surface of the slab thoroughly.
Next, unfurl 6-mil polyethylene sheeting over the floor to provide moisture protection. When laying the next row of sheeting, be sure to overlap the existing row by 6 inches, and then tape the seams. Adhere the edges of the polyethylene to the slab using a caulking gun; this holds the sheeting in place and prevents slippage.
Lay out pressure-treated 2-by-4 sleepers at the edges of the room, abutting their ends and fastening them to the slab with 2 1/4-inch masonry nails (a power-actuated nail gun is an option). Mark these perimeter sleepers for additional sleepers that will be centered every 16 inches and laid out in one direction across the room.
Affix these sleepers to the slab, nailing at the end of each board and approximately every 4 feet thereafter. Check for level often as you go through this process. Shims can be added where needed to ensure a level surface.
Fit 1 1/2-inch-thick rigid foam insulation between the sleepers; strips should be about 12 1/2 inches wide. Always measure for an exact fit so as not to compress the insulation, which can compromise its performance.
Place 3/4-inch plywood sheets across (not parallel to) the sleepers. Start alternate rows with half sheets to stagger the joints. Drive in 6d nails every 6 inches where the ends of the panels meet the sleeper supports and every 12 inches into sleepers under the panel centers.
Once the plywood surface is laid out, you can finish with the flooring material of your choice. For more on insulating foundations, see How to Seal (Caulk) Around Your Home’s Foundation.
